1. Field of the Invention
The present invention generally relates to circuitry for protection of data stored within volatile devices, and more specifically, relates to circuitry capable of reacting to a fast negative transition of a supply level from a primary power surce for initiating a switchover from the primary power source to a secondary power source.
2. Description of the Related Art
Volatile devices require power to maintain stored data. If power is removed from the volatile devices, the stored data is lost. Such volatile devices include volatile memory, clocks, and any other device that will lose the data upon the loss of power.
Conventional systems generally contain power source switchover circuitry for detecting that a supply level from a primary power source decreases below a threshold voltage and for initiating a switchover to a secondary or backup power source. The switchover to the secondary power source maintains power to the volatile elements to avoid loss of the stored data. However, the power source switchover circuitry utilized to switch from the primary power source to the secondary power source in conventional systems is designed to be stable over temperature and process corners, and not designed to respond to a fast ramp-down or negative transition of the supply level of the primary power source.
System designers have recently been interested in designing systems capable of performing a fast ramp-down of the primary power source. This presents a problem as the power source switchover circuitry in conventional systems cannot initiate a switchover from the primary source to the secondary power source for a fast ramp-down of the supply level of the primary power source (i.e., the fast ramp-down of the supply level is not detected by the power source switchover circuitry).
To overcome the shortcomings of the power source switchover circuitry in conventional systems, a circuit and method for responding to a fast ramp-down of a supply level from a primary power source and for performing a switchover from the primary power source to a secondary power source to preserve stored data in memory elements is presented. The circuit and method, according to the principles of the present invention, allow a system designer to account for ramp-down of the primary power source at faster rates than in conventional systems.
One embodiment of the present invention includes an integrated circuit for providing a switchover from the primary power source to the secondary power source. The integrated circuit includes a power source switchover circuit for detecting the supply level of the primary power source and a voltage threshold of the power source switchover circuit. The power source switchover circuit has a first input terminal selectively coupled to the primary power source and a second input terminal selectively coupled to the secondary power source. A comparator is coupled to the power source switchover circuit for indicating that the supply level of the primary power source decreases below the voltage threshold. The integrated circuit further includes a forced power source switchover circuit for detecting that the supply level of the primary power source drops below a predefined threshold level. A switchover circuit on the integrated circuit initiates a switchover operation based upon a switchover indication from the comparator or upon an indication that the forced power source switchover circuit detects that the supply level being received from the primary power source drops below the predefined threshold level.
Another embodiment according to the principles of the present invention is a method for performing a power source switchover from the primary power source to the secondary power source. The method includes detecting What a supply level being received from the primary power source decreases below a predefined threshold level from a steady-state operating level, where the decrease occurs faster than a predetermined negative rate of change. The method asserts a signal indicating to switch from the primary power source to the secondary power source upon detecting that the supply level being delivered from the primary power source has decreased below the predefined threshold level. A detection of the signal indicating to switch from the primary power source to the secondary power initiates a switch from the primary power source to the secondary power source.
Another embodiment according to the principles of the present invention is a system having at least one volatile device for maintaining data, a forced power source switchover circuit for detecting a drop in a supply level below the predefined threshold level from the primary power source, and a switchover circuit for asserting a switchover from the primary power source to a secondary power source to prevent the volatile device from losing data upon power being removed from the volatile device. The forced power source switchover circuit reacts to the supply level transitioning from a steady-state operating level to a predefined threshold level, the supply level transitioning at a rate faster than a predetermined negative rate of change. The system may be a memory system, a computing system, a communications system and/or a clock system. The volatile device may be a memory device or a clock device.